1. Technique Field
The disclosure relates to an image display apparatus, and in particular relates to an electrically switchable light-modulating cell thereof and a method for driving the same.
2. Description of the Related Art
In 1875, Dr. Lippman discovered an electrowetting phenomenon. Thereafter, research and several applications utilizing the electrowetting technology were developed, e.g. displays, optical filters, optical fibers, adaptive lenses, adaptive passive devices such as capacitors, lab-on-chips, and spray coating. As shown in FIG. 1, a polar liquid (no applied voltage) on a hydrophobic dielectric layer appears to be contracted, thereby minimizing its Gibbs free energy. In a display with pixels smaller than 200 μm, the liquid droplet has a surface tension over 10,000 times that of gravity. In other words, the liquid droplet is stable in any direction. When a voltage is applied to the polar liquid, charges are distributed on the dielectric layer to attract the polar liquid. As such, the polar liquid is spread out to minimize the Gibbs free energy thereof. If a voltage applied to the polar liquid is larger, the polar liquid is spread out on the hydrophobic layer more. The manner of applying voltage to the polar liquid will be influenced by certain factors, e.g. the voltage, the thickness of the hydrophobic dielectric layer, the dielectric constant of the hydrophobic dielectric layer, and the surface tension between the polar liquid and the hydrophobic dielectric layer.
FIG. 2 shows curves of contact angles (between a polar liquid and a dielectric layer) versus voltages applied to the polar liquid. Although the theoretical curve can be calculated by the Young-Lippmann equation, the real curve should be obtained by experiments. In addition to the electric potential applied to the polar liquid, the surface tension, the interface interaction, the temperature, and the force of gravity also influence the behavior of the polar liquid. For effectively applying the electrowetting phenomenon, the characteristics of the polar liquid (such as the tilt angle of the liquid surface) should be exactly controlled. For achieving the above requirement, the disclosure provides an electric potential difference generator (e.g. discrete electrodes) to precisely control the liquid characteristics.